1. Field of the Invention
The present invention relates to elastic wave devices for use in resonators, bandpass filters, and the like, and more particularly, to elastic wave devices that use piston modes.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2011-101350 discloses an elastic wave device that uses a so-called piston mode. In Japanese Unexamined Patent Application Publication No. 2011-101350, the acoustic velocity at a top side portion of electrode fingers in an IDT electrode is lowered compared with the acoustic velocity in a cross-over portion of electrode fingers. This activates a piston mode and, as a result, can suppress ripples by a transverse mode.
Japanese Unexamined Patent Application Publication No. 2013-518455 also discloses an elastic wave device that uses a piston mode.
In transmission filters of cellular phones and the like, there is a need for enhancement of electric power handling capability, an improvement of IMD, and the like. To meet such needs, a resonator is often series-divided into multiple stages.
In the case that an elastic wave resonator described in Japanese Unexamined Patent Application Publication No. 2011-101350 is series-divided, there is a problem of increase in size in a direction orthogonal to an elastic wave propagation direction. Hereinafter, this problem is described with reference to FIG. 6.
FIG. 6 is a schematic plan view depicting an electrode configuration when the elastic wave device described in Japanese Unexamined Patent Application Publication No. 2011-101350 is series-divided. An elastic wave device 1000 is divided into a first elastic wave resonator unit including a first IDT electrode 1001 and an elastic wave resonator unit including a second IDT electrode 1002. In FIG. 6, a right side portion of the electrode configuration schematically illustrates acoustic velocities of elastic waves in respective regions. For example, the acoustic velocity in a center region of the first IDT electrode 1001 is V1, and the acoustic velocities in low acoustic velocity regions at both sides of the center region are V2A and V2B. The acoustic velocity is higher as the position of its indicator moves farther to the right.
An inter-stage busbar 1007 is disposed between the first IDT electrode 1001 and the second IDT electrode 1002, and this inter-stage busbar 1007 connects in series the first IDT electrode 1001 and the second IDT electrode 1002.
The regions of the acoustic velocities V2A and V2B, which are provided at both sides of the center region, are the low acoustic velocity regions. In the low acoustic velocity regions, an acoustic velocity lowering film 1005 is provided on electrode fingers 1003 and 1004. This slows down the acoustic velocity. The second IDT electrode 1002 has a configuration similar to the above.
As is evident from FIG. 6, a high acoustic velocity region whose acoustic velocity is V3B, the inter-stage busbar 1007, and a high acoustic velocity region whose acoustic velocity is V13A are needed in a portion connecting the first IDT electrode 1001 and the second IDT electrode 1002. In other words, the length of a dimension Wc in FIG. 6 needs to be made longer than a certain value. Thus, in the elastic wave device 1000, the dimension in a direction along which the electrode finger extends becomes longer, thereby making it difficult to downsize.